1. Field of the Invention
The claimed invention relates generally to well drilling and servicing equipment and more specifically to portable rigs which have a mast which is horizontal when being moved and is vertical when in use and which is used to lower and raise objects into and out of well bores drilled in the earth.
2. Setting of the Art
Production wells, i.e. wells which are drilled in the earth to produce oil and gas, must be "worked over" or serviced from time to time which includes replacing of downhole equipment or the lowering or raising of objects into the well bore such as tubing, rods, tools and so forth. For example, if the production string of casing which is set in the well bore to keep the production fluid from escaping is damaged or leaking, it may be necessary to pull all or part of the casing from the well bore and replace it with a new string or partial string of casing or pipe. Most often an artificial means of raising the fluid from the bottom of the well bore is used. One such system involves the use of sucker rods which are hung in a well and reciprocated to drive a downhole pump. Sometimes the sucker rods must be pulled to replace damaged or broken rods, worn or damaged pumps, etc. Another system is the gas lift. However, frequently the gas lift valves are not working properly and therefore, it may be necessary to pull the tubing and exchange the gas lift valves prior to placing back into the well. Also, when the tubing, which is hung in the well bore through which the well fluids are produced becomes plugged with sand, it is necessary to insert a small diameter work string into the pipe to remove or flush out the material clogging the flow of oil through the pipe. Other remedial service operations include gravel packing, fishing jobs, plug backs and so forth.
When such service operations become necessary it is quite common to use a portable workover rig which can be moved to the well site and set up. Generally, these rigs consist of a derrick or mast which supports pulleys or block and tackle arrangements operable to pull the pipe string from the well bore as well as lower the pipe string and workover tools into the well itself. These workover rigs normally have what is called a mainline drum about which a heavy cable is wound and the free end is connected over the crown block at the top of the well bore and is connected to what is called a traveling block. By rotating the drum the traveling block can be raised or lowered with the drilling mast as is necessary. Another drum is called a sandline drum (not used with the traveling block) and which has a line wound about it and up over the crown block and is then connected to an object such as a swab mandrel which is to be lowered into the well bore to a selected depth. Nearly all of the conventional workover rigs have these mainline drums and the sandline drum driven by a chainlink drive. A clutch is then necessary to engage and disengage the drive chain. With these chain driven systems it is difficult to make the drum rotate in a controlled manner when lowering heavy objects into the well bore. With the chain drive you have to let it freewheel by releasing the clutch and depend entirely on the drum brake.
Another workover operation is the removing of objects which become stuck in the well bore. The tubing is pulled and stretched until the load limit is reached. The clutch is released allowing the tubing to contract and is then caught with the mechanical drum brake, thus imparting a shock down the tubing which may free it from its restraint.